Monday, 27 April 2015

It is well established that there is a functional columnar structure repeated throughout the cortex across species (the concept received significant attention after a study by Mountcastle in 1957 [1]; see [2] and [3] for reviews). This suggests a universal cortical algorithm, which is fascinating if you are trying to understand the computational underpinnings of the mind from a biological perspective, or like us, to implement AI aided by learnings from neural principles.

Assuming that there is in fact a universal cortical algorithm, we would like to define the column, and understand its function through first understanding how it is composed. It’s possible to find a columnar structure at multiple scales, which has produced a range of terms in the literature, from micro-column to mini-column, to macro and hyper-column. Many studies have approached it differently, leading to inconsistent definitions. It can be really confusing to switch between sources that make different assumptions. This has caused us to confuse ourselves as well, alternating between different internal definitions over time.

“Although the anatomical and functional columnarity of the neocortex has never been in doubt, the size, cell composition, synaptic organization, expression of signaling molecules, and function of various types of “columns” are dramatically different. Columns could be defined by cell constellation, pattern of connectivity, myelin content, staining property, magnitude of gene expression, or functional properties. For example, there are ocular dominance columns, orientation columns, hypercolumns, and color columns, to mention only those described in the primary visual cortex (12), that differ from each other as well as from the columns of the alternating callosal and ipsilateral projection in the frontal lobe (8) or various minicolumns advocated by Szentgahotai (7), Eccles (9), Buxhoeveden and Casanova (10), and a more recent detailed reconstruction of barrel field columns by Sakmann and colleagues (13) and their visibility in vivo by neuroimaging (14). The only connections between these diverse structures and concepts is that they refer to the vertical or radial columnar organization of its elements as opposed to the horizontal or laminar organization that is more explicit in histological preparations of the mature neocortex. Thus, the term cortical “column” is used in so many ways that it can be very confusing to the nonspecialist if not more precisely defined.”
See original paper for the quoted included citations.

This quote is from an editorial in PNAS, where Rakic discusses a paper in that edition which disproves a long held belief that the anatomy of the cortex is uniform. This may appear to undermine the notion of a universal cortical algorithm, but it is focussed on anatomical features rather than the presence of functional columns.

So we were very keen to clearly define a nomenclature consistent with the most accepted definitions, and stick to that. We discovered a paper by Rinkus from 2010 [5], which is part of a whole edition of Frontiers on the cortical column. It’s an excellent resource, highly recommended. There are many good papers, but I found Rinkus’ most useful for the best definition of cortical columns that I’ve read. That’s because it is nuanced, based on function, and part of an understanding (or at least proposal) of the algorithm of a region of neocortex. From this, we’ve created our canonical definition of mini and macro columns. To be published in the next blog post.